1. Field of the Invention
The invention relates to a surface wave filter with a slab-like substrate of monocrystalline piezoelectric material, particularly lithium niobate. On one substrate side of the slab of piezeoelectric material are applied input and output transducers which optionally may be coupled by means of a coupler. A plastic and/or elastic damping layer for suppressing reflected waves is applied. This damping layer covers, at least in some regions, this substrate side except for the area which is bounded by the transducers, the coupler, the ends of the electric contact runs and the zones between the transducers and the coupler.
2. Description of the Prior Art
In manufacturing the surface wave filters with a substrate of monocrystalline piezoelectric material, a metal layer is customarily vapor-deposited on the substrate, from which the transducers and, if necessary, also the couplers and the contact runs with their ends, are worked out, for instance by means of a photo-etching technique. The so patterned substrate is cemented on a supporting metal sheet and the ends of the conductor runs are electrically connected to the contact legs of the filters by bonding wires. The system is then inserted into a metal or plastic housing and is filled with casting compound.
If electrical signals are fed to the input transducer 2 (see, for instance the example shown schematically in the top view in FIG. 1 and in a side view in FIG. 2, for a known damped surface wave filter with coupler), this transducer 2 emits in the same rhythm surface waves in different directions. The couple 4, which consists, for instance of strip-shaped conductor runs extending parallel to each other, picks up a part of the emitted waves from transducer 2, specifically that directed toward the coupler 4, and passes it on in the direction toward the output transducer 3 which reconverts it into electrical signals. Reflected waves which are generated at cut edges of the substrate 1 and which are also caused by crystal structure faults in the substrate 1 and are therefore time-delayed and are interfering are suppressed partially by technological measures. For this purpose, a damping compound 9, consisting of a heat adhesive is melted onto the surface between the substrate end face edge and the transducer 2 and similarly a damping compound 10 is melted onto the surface between the other substrate face edge and the transducer 3. In this process, however, only a relatively small area parallel to the cut edges of the chip surface can be damped. Consequently, despite damping masses 9 and 10, undesired reflections of waves which have an interfering effect on the transducers 2 and 3 and the coupler 4 which are all applied to the one substrate side of a substrate 1, are unavoidable.